This invention relates to seismic geophysical surveying and is directed to surveying by the use of cyclically varying seismic waves generated by a vibration generator. Specifically, the invention uses a modulated vacuum to transfer the vibrations to the earth and does not require either a reaction mass to generate the vibrations or a hold-down mass to keep the apparatus in contact with the earth's surface.
The most common method of generating seismic signals is probably by means of an explosive. In recent years, however, several methods of seismic surveying have been developed which utilize a vibrating energy source, such as a mechanical vibrator. A typical, commonly used mechanical vibrator, for example, has a gross weight of about 31,260 pounds (about 22,000 pounds of which is used as hold-down weight), and has a peak force rating of about 15,000 pounds. This unit has a reaction mass of 3,800 pounds, which is hydraulically accelerated alternately up and down to generate the 15,000-pound force (30,000 pounds peak to peak). The acceleration of this relatively large mass requires heavy equipment, and the complete unit is expensive and heavy. The unit is truck-mounted, but it is not as portable as would be desired as there are many areas inaccessible to large trucks. Even though the system used for such vibrators is very large, the seismic waves generated are somewhat limited and often multiple units are used with their outputs synchronized by systems such as described in U.S. Pat. No. 3,698,508, issued to Landrum, on Oct. 17, 1972. The mechanical vibrator exerts a relatively high pressure on the earth's surface. In the aforementioned unit, a maximum of 37,000 pounds of force (15,000-pound peak force plus 22,000 pounds of hold-down weight) is exerted on a pad of about 3,000 square inches, and the resulting 12.3 pounds per square inch frequently leaves an imprint several inches deep in the earth's surface, which imprint is often undesirable. In some cases, of course, the earth's surface is too soft to withstand the 12.3 pounds per square inch pressure, and the mechanical vibrator cannot be used.
Systems have been proposed to reduce the mass somewhat by eliminating either the reaction mass or the hold-down mass. U.S. Pat. 3,701,968, issued to Broding, on Oct. 31, 1972, and U.S. Pat. 3,792,751, issued to Fair, on Feb. 19, 1974, are examples of systems using changes in air pressure (rather than a reaction mass) to generate vibrations. Both systems require a hold-down mass and use the mass of the apparatus as the hold-down mass. U.S. Pat. No. 2,910,134, issued to Crawford, on Oct. 27, 1959, is an example of a system which uses a reaction mass, but which uses a partial vacuum (rather than a mass) to provide the hold-down force. Crawford's system results in rather high concentrations of pressure on the earth's surface as the vibration forces and the hold-down forces are both transferred to the earth through the shell of the structure.